C++ Program to evaluate a postfix expression

by Arun Vishnu · Published November 28, 2008 · Updated August 28, 2011

C++ Program to evaluate a postfix expression.

/**************************************************************	
	Author: Arun Vishnu M V
	Web: www.arunmvishnu.com
	Description: C++ Program to evaluate a postfix expression.
***************************************************************/
#include;     // For isdigit()
#include;    // For clrscr()	
#include; // For exit()
#include;  // For gets()
#include;  // To find the power

# define MAX_STACK 150  

class stacks
{
   int value,item,top;
   int stack[MAX_STACK];
   public:
      stacks()// Constructor
      {
	 top=(-1);
      }
      void push(float);
      float pop();
};

// Function to PUSH the vale to the stack

void stacks::push(float value)
{
   if(top==MAX_STACK)
   {
      cout<<"Stack is full!\nOverflow!";
      getch();
      exit(0);
   }
   else
   top++;
   stack[top]=value;
}

// Function to POP the value from the stack

float stacks::pop()
{
   if(top<0)
   {
      cout<<"Stack is empty!\nUnderflow!";
      getch();
      exit(0);
   }
   else
   item=stack[top];
   top--;
   return(item);
}

// MAIN Function

int main()
{
   clrscr();
   stacks stk1;  //Object of stack
   char expression[150],oper_string[8],ch;
   oper_string[0]='\0';
   float value,operand1,operand2,val;
   int pos=0,p=0,iv;
   cout<<"Enter a postfix expression: ";
   gets(expression);
   while(expression[pos]!='\0')
   {
      ch=expression[pos];
      
      //Converting the operaand string to float

      if((isspace(ch)) &&(oper_string[0]!='\0'))
      {
	 oper_string[p]='\0';
	 stk1.push((atof(oper_string)));
	 oper_string[0]='\0';
	 p=0;
      }
      else if(ch==' ')
      {
	 pos++;
	 continue;
      }
      else if((isdigit(ch)))
      {
	 oper_string[p]=ch;
	 p++;
      }
      
      // End of conversion
      
      else
      {
	 operand1=stk1.pop();
	 operand2=stk1.pop();
	 switch(ch)
	 {
	    case '+':
	       val=operand2+operand1;
	       break;
	    case '-':
	       val=operand2-operand1;
	       break;
	    case '*':
	       val=operand2*operand1;
	       break;
	    case '/':
	       val=operand2/operand1;
	       break;
	    case '^':
	       val=pow(operand2,operand1);
	       break;
	    default: break;
	 }
	 stk1.push(val);
      }
      pos++;
   }
   cout<<"\nValue= "<< stk1.pop();
   getch();
   return(0);
}